1. Field of the Invention
This invention is in the field of semiconductor films which are useful in photovoltaic cells.
2. Description of the Prior Art
Photovoltaic cells have been developed for generating electrical energy directly from sunlight. Typically, these cells have been based on a semiconductor layer having an ohmic contact on one side and a rectifying contact, such as a p-n junction or a Schottky barrier, on the other side. Many semiconductors have been tested for this application and it has been found that silicon has desirable properties for use in photovoltaic cells.
Using silicon wafers, photovoltaic cells have been prepared with power efficiencies of up to about 18%. The cost of such cells, however, is very high, and such cells, in fact, have been considered to be prohibitively expensive. Because of this, silicon solar cells have only been used to any significant extent in applications where cost is not a controlling factor, such as on space vehicles or power sources in remote areas.
This high cost of photovoltaic cells is due in large measure to the requirement for crystal perfection which is necessary for high-efficiency operation and to the elaborate procedures involved in wafer preparation. Cell efficiency is lowered by a high density of crystallite grain boundaries which are present in polycrystalline silicon of small grain size. In order to eliminate as many grain boundaries as possible, the typical fabrication of silicon wafers for photovoltaic cells starts with the pulling of a high grade single crystal silicon boule which is then cut into wafers over 400.mu.m thick. These are lapped, polished and mounted, after which a junction is formed by indiffusion of a dopant. The photovoltaic structure is then fabricated.
This procedure for the fabrication of solar cells is necessary to achieve the highest possible efficiency presently available. This is important in space applications, where efficiency is vital and cost is less important. For terrestrial applications, however, cost is the major factor, and some decrease in efficiency is acceptable if it permits a significant cut in cost.
One direction presently under study for the production of inexpensive silicon photovoltaic cells is the pulling of silicon ribbon in the EFG process. See, for example, Chalmers, B., LaBelle, Jr., H. E. and Mlavsky, A. I., J. Cryst. Growth, vol. 13/14, p. 84 (1972). This method bypasses the expensive cutting and polishing required in silicon wafer processing, and directly produces ribbon about 400.mu.m thick. However, this scheme is not perfected, and is not yet capable of producing high quality material in a continuous process for long periods of time.
An alternative approach to the production of inexpensive photovoltaic cells is the use of thin film silicon deposited on a suitable backing. Such films having a thickness of about 10.mu.m, for example, when suitably processed, are capable of producing electrical power with an efficiency potentially up to about 80% of that obtained with silicon single crystal wafer cells, or well over 10% efficiency. Such efficiencies are considered adequate for terrestrial applications provided that the cost of the cells is markedly reduced from that of conventional cells. Thin film silicon photovoltaic cells, with a thickness of about 10.mu.m, compared to about 400.mu.m for wafer photovoltaic cells, would allow considerable saving in material. Moreover, such films could be deposited in the final step of a process for the purification of silicon, thereby avoiding the cost of crystal pulling, and the cutting and polishing of wafers. However, the vital remaining step which must be taken before silicon films can be used in the fabrication of efficient photovoltaic cells is the treatment of the films to produce large grain crystalline material.
Attempts have been made to fabricate semiconductor films having large oriented crystallites for use in photovoltaic cells. Chemical vapor transport, vacuum deposition, high temperature thermal crystallization, and electron beam heating, for example, have all been tried. Most of these have inherent disadvantages or have shown only limited capability to produce the desired films. Vapor deposition and electron beam heating also require high vacuum conditions which add to the complexity and expense of film production.
Recently, attempts have been made to utilize lasers for the improvement of polycrystalline silicon films. It has been reported in the literature, for example, that a two-step processing technique employing transient laser heating and melting can be used to produce isolated regions of larger-grain polycrystalline silicon in films of fine-grain silicon on substrates such as silica. See Laff, R. A. and Hutchins, G. L., IEEE Transactions on Electron Devices, vol. ED-21, no. 11, Nov. 1974, p. 743. In the Laff et al. technique, a Gaussian-focused CW argon-ion laser beam of moderate power (1.5W) was swept along the surface of a 1.mu.m thick film of fine grain CVD-deposited silicon at a velocity of about 10 cm/sec. This beam was modulated as it was swept so that only local regions along the track were illuminated. This caused localized heating of the film to about 1000.degree. C for about 10.sup.-4 sec. which produced recrystallization and made heated areas selectively etch resistant as compared to surrounding fine grain areas. Repetitive stepwise deflection of the swept modulated beam was employed to obtain maskless fabrication of X-Y arrays of isolated silicon islands, resulting in crystallite grain sizes of about 1.mu.m. A second and slower application of the laser beam applied to these isolated islands caused local melting with zone length L &lt; 50.mu.m and time of molten state t .ltorsim. 10.sup.-3 sec., which resulted in crystallite grain sizes of up to about 5.mu.m.
Despite some enlargement of silicon grain sizes, however, the Laff et al. procedures are not sufficient for large scale, inexpensive production of silicon films for photovoltaic cells. The argon-ion laser used, for example, is very inefficient having an overall power efficiency of only about 0.05%. Also, because of the high absorption coefficient of silicon at the frequency of an argon-ion laser, heating is limited to the surface layer which probably makes this laser unsuitable for preparing silicon films of sufficient thickness for solar cells.
Thus, despite the enormous amount of research directed at developing new production techniques for semiconductor films useful for photovoltaic devices, none of those developed heretofore have proven to be entirely satisfactory. There is still a great need for improved techniques.